Fibers for optical transmission are generally composed of a special optical glass which is a brittle material. To improve its inadequate mechanical strength, it is generally provided with a lacquer-like covering and a plastic sheath. However, the optical fiber cable thus obtained, which is composed of a glass fiber core and surrounding sheaths, still has various shortcomings. For example, breakages often occur in the cores, and their transmission properties are impaired by external pressures, impacts or bending stresses. Other suitable sheathing materials have thus been sought for some time in order to overcome these deficiencies.
Optical fibers composed of a core and a sheath made up from one or more layers are known. For example, they may be produced by the process described in U.S. Pat. No. 3,980,390 in which a glass fiber spun from a melt index is provided, before it comes into contact with other solid substances, with a resin coating on which a further coating composed of a thermoplastic resin composition is applied by melt extrusion. The first coating of the resin composition, hereinafter called the primary coating, is applied to the surface of the fiber directly after it has been spun and serves to assist in maintaining the original strength of the glass material. The subsequently applied coating is composed of a thermoplastic resin composition, hereinafter called the secondary protective layer, which is applied by extrusion and provides protection against mechanical stresses, moisture, ultraviolet radiation, etc. An optical cable produced by this process, with a sheath composed of two layers, is shown in FIG. 1 of the accompanying drawings. This Figure shows glass fiber core 1, primary coating 2, and secondary protective layer 4.
It is already known from D. Gloge et al "Optical Fiber Packaging and Its Influence on Fiber Straightness and Loss (BSTJ, Vol. 54, 1975, pages 245 to 262) or J. Geisler et al. "Optical Fibers" (Appl. Technical Ser. V/S 120 uf. Pergamon Press, 1986), that the transmission properties of optical fibers are subject to marked variations as the result of microbending. The resultant transmission losses are usually given in dB/km.
There have, therefore, been attempts to improve the double-layered structure of the sheath of the optical fiber of FIG. 1. A typical example is shown in FIG. 2, wherein there is provided, between primary coating 2 and secondary layer 4, buffer layer 3 of a material which can absorb external stresses or strains and is composed, for example, of polyisobutenes, certain gels, foamed plastics, an ethylene/vinylacetate copolymer, a conventional commercial silicone resin, or a rubber-like material.
In a further industrial design, the internal diameter of the secondary protective layer is larger than the external diameter of the primary coating to produce a space between the two coatings. A cable of this type is shown in FIG. 3 of the accompanying drawings.
In a still further industrial design, the secondary layer is designed as a flat strip in which the optical fibers are arranged next to one another in parallel rows (see FIG. 4). The advantage of this design resides in the fact that a large number of optical guides lie next to one another in a strip and several such strips can be arranged on top of one another to allow a flexible method of construction.
The optical cables of types shown in FIGS. 1 to 4 are distinguished in that the fiber core is mechanically isolated, by the primary coating and optionally the secondary protective layer, from external mechanical forces as well as internal or external stresses caused by the differing coefficients of thermal expansion of the various materials of which the optical cable is composed. Investigations have shown that, in contrast to cables according to FIGS. 1 and 4, a lesser increase in transmission losses resulting from external pressure or lower temperatures is observed in cables according to FIGS. 2 and 3. Owing to the space between the primary coating and secondary layer, the cable according to FIG. 3 also has high resistance to microbending which occurs as the result of the external force or internal thermal stresses. Shrinkage of the secondary layer in the longitudinal direction may lead to coiling of the optical fibers without causing transmission losses.
Thermoplastic resins which may be extruded as melts are known for producing the secondary protective layer. Polyamides, polyesters, polyolefins, and fluorine polymers, in particular, have been used, as these materials are easily extruded, are very resistant to weathering, and have high mechanical strength. It is particularly advantageous to use polyamides which have a relatively low coefficient of thermal expansion and low water absorption capacity which are also those used for the coating of electric wires.
Various investigations have shown that polyamide-12 resins can be used successfully for producing the secondary protective layer for optical fibers. Such fibers are substantially free from variations in the transmission losses occurring as the result of microbending; i.e. those resulting from the application of the secondary protective layer by extrusion and those caused by external stresses which occur during sheathing, installation, or cable laying. Optical fibers which have a coating of polyamide 12 and related copolyamides whose moduli of elasticity are between 200 and 2200 N/mm.sup.2 at room temperature are known from DE-OS 25 12 312; 27 23 587; 27 24 155; and 31 44 182; DE-PS 29 14 555; and JP 14 82 10/78.
In addition to their resistance to microbending, the optical fibers must also have stable transmission properties at temperatures of -30.degree. to +60.degree. C. and must maintain their properties even if they are used, for example, as underwater cables where they are subjected to high water pressures.
This effect can be achieved if a sheathing material having a high modulus of bending elasticity is selected; this will ensure a sufficiently high resistance to transverse pressure. However, suitable materials frequently have viscosities, such that production of the secondary protective layer cannot be carried out at a satisfactory rate by extrusion. Materials which have a high level of the desired qualities and, at the same time, allow high processing speeds are therefore desired for inexpensive and industrially simple production.
Therefore, an object of the invention is to overcome the above-mentioned disadvantages of such cables and, more particularly, to improve (1) the viscosity-dependent irregular thicknesses of the applied plastic coatings, (2) inadequate resistance to pressure and to buckling, (3) excessive water absorption capacity, (4) reduced resistance to microcracks with a small bending radius, (5) defective dimensional stability due to after-crystallization of the polymer, and (6) inadequate mechanical strength. All of the foregoing increase the transmission loss and also increase the cost of production owing to the excessively low extrusion rate of the known materials.